In the coatings area, pigmented coating compositions are commonly employed to obtain a final finish that possesses both protective and aesthetic qualities. Such coating compositions are formed from a mixture of durable film-forming resins plus finely dispersed pigments. To incorporate the pigment into the coating composition, the pigment is usually dispersed in a resinous dispersing vehicle, typically by a milling or grinding process. The resultant pigment dispersion is combined with the film forming resinous binder to form tile coating composition. To ensure proper color development and appearance, it is important that the pigment or pigments employed be well dispersed in the dispersing vehicle and throughout the coating composition. As such, the choice of dispersing vehicle and the process by which the pigments are dispersed in that vehicle is of critical importance.
As mentioned above, for coating compositions, the pigment dispersions are, for the most part, produced by a dry grinding process. This process typically involves several steps including first drying the aqueous pigment slurry in which the pigment is prepared. This is usually done by the pigment manufacturer to produce a dry agglomerated pigment, which is then ground or milled by the coatings supplier with the dispersing vehicle to break up the agglomerates and to disperse the pigment in the dispersing vehicle. Such a dispersion method requires a high expenditure of energy and can also be time consuming. In addition the resulting pigment dispersion has a relatively low pigment to dispersion vehicle ratio which can result in low tint strengths. The relatively high levels of dispersion vehicle can affect final coating properties and as a result many different pigment dispersions or tint lines must be prepared for various coating lines. For example, a pigment dispersion useful in a thermally cured coating would not necessarily be usable with an air dried coating or another type of thermally cured coating.
Another method useful for producing pigment dispersions, primarily for use in inks, is the "flushing method". This method involves treating a pigment presscake, i.e., a concentrated aqueous slurry of pigment obtained in the pigment manufacturing process, with a resinous pigment dispersing vehicle, commonly referred to as a flushing vehicle, under shearing conditions to displace water on the pigment surface by the resinous vehicle. With the flushing method, a stable pigment dispersion can be produced without the intermediate steps of drying the pigment slurry and milling the pigment to break up pigment agglomerates and disperse the pigment in the dispersion vehicle. Relative to dry grinding, or milling, flushing offers the potential for obtaining a pigment dispersion in less time and with less energy. However, the pigment flushing method has not been widely applied in the area of exterior durable coatings. Flushing vehicles that are commonly used in the ink industry, i.e., alkyd resins, do not weather well in finishes that must endure prolonged exterior exposure.
Therefore it would be desirable to have a durable pigment dispersion vehicle which could be used as a flushing vehicle in the pigment flushing process to provide pigment dispersions which could be used across numerous coating lines.